My kids had highlights and streaks done at a salon when they were about 8 or 9 y.o. They had fun trying to dye with Kool-Aid at about the same age. Their hair is dark though, so it didn't take very well.
They were both about 16 y.o. before they tried a complete dye job, with first bleaching out their natural colour and then using the dye. That's about the age when I figure it's really up to them. At that point, I've warned them about the problems of chemicals, over-processing and so on. They both used dyes from the pharmacy at about that age. DS had various coloured (at different times he tried bright blue, dark green and red - not all those colours at the same time) liberty spikes in his last couple of years of high school. A couple of years ago, DD had a purple underlayer. Last year, when she was 16 y.o, she had bright pink hair for most of the year.
They are 20 and 17 y.o. now and both have their hair back to their natural colours now.
I was about 40 or so when I first dyed my hair. I dyed it to cover grey, so it was basically getting it back to my natural colour. I gave up on that about 6 years ago and let the grey come in.